1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member; an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
There is widely known a fixing device that includes a fixing roller equipped with a halogen heater inside thereof and a pressure roller disposed opposite the fixing roller, thereby defining a fixing nip. The recording medium bearing the unfixed toner image is conveyed to the fixing nip where heat and pressure are applied thereto, and the unfixed toner image is fixed. This fixing method is known as a fixing method using a heat roller.
Another known fixing method is a belt fixing method, in which an endless-loop fixing belt is wound around and stretched between a heating roller equipped with a halogen heater inside thereof and a fixing roller pressed by a pressure roller through the fixing belt. The pressure roller and the fixing belt define a fixing nip in which heat and pressure are applied to the recording medium and the unfixed toner image thereon is fixed.
In either method, because the toner image fused on the recording medium contacts the fixing roller or the fixing belt, the fixing roller or the fixing belt is generally coated with fluorocarbon resin to facilitate separation of the recording medium from the fixing roller or the belt. In addition, a separation pawl is used to separate physically the recording medium from the fixing roller or the fixing belt. Disadvantageously, however, the known separation pawl contacts the fixing roller or the fixing belt and consequently the separation pawl may damage the surface of the roller or the belt. When this happens, an output image has undesirable streaks appearing therein.
To address such a problem, a generally-known monochrome image forming apparatus employs a fixing roller made of a metal roller coated with Teflon (registered trademark). In this configuration, the fixing roller is prevented from getting easily damaged even when the separation pawl contacts the fixing roller, thereby enhancing durability.
By contrast, in a case of a color image forming apparatus, the fixing roller has a surface layer made of silicone rubber (generally, a PFA tube with a thickness of some tens of microns is used) coated with fluoride, or applied with oil to enhance color development. In this configuration, the surface layer is relatively soft and hence can be damaged easily by the separation pawl. Accordingly, color image forming apparatuses in recent years hardly employ such a separation pawl that directly contacts the fixing belt to separate the recording medium therefrom, but instead employ a so-called contact-less separation method.
Various contact-less separation methods have been proposed. For example, 1) a small gap (approximately 0.2˜1.0 mm) is provided between the fixing roller/belt and a separation plate extending parallel to the fixing roller/belt, known as a contactless separation plate method. Or, 2) a small gap (approximately 0.2˜1.0 mm) is provided between the fixing roller/belt and a plurality of separation pawls, which are disposed with a predetermined interval between each other, known as a contactless separation claw method. Alternatively, 3) the recording medium is separated from the fixing roller/belt using the resilience of the recording medium itself and the elasticity of a curved portion of the fixing roller/belt, known as a self-stripping method.
Common to all of the above-described approaches is a gap between a guide member for guiding the recording medium to the end of the fixing nip and the fixing roller/belt. When conveying a thin recording medium and/or a recording medium with little margin at the leading end thereof in the fixing nip, or when conveying a recording medium with an image such as a photograph, the recording medium tends to stick to the fixing roller/belt and remains adhered thereto, passing through the gap. As a result, the recording medium is rolled onto the fixing roller/belt, and/or paper jam occurs when the recording medium comes into contact with the separation plate or the separation pawl.
In view of the above, JP-2008-102408-A proposes blowing compressed air from a nozzle against an appropriate position for separating the recording medium from the fixing roller/belt.
Such a sheet separation mechanism includes an air pressure piping system to regulate the compressed air projected from a compressor to the nozzle. Using the compressed air, the recording medium is separated reliably from the fixing roller/belt without damaging the fixing roller/belt. Furthermore, this configuration is advantageous because the compressed air can be used to clean detection surfaces of detectors such as a temperature detector for detecting the temperature of the fixing member and a detector for detecting the presence of the recording medium by blowing the compressed air against these surfaces.
The related-art sheet separation device using the compressed air includes an air filter, an air tank, and a pressure adjusting valve, air pressure members such as an electromagnetic valve and a nozzle, and pipes connecting these parts, constituting the air pressure piping system. The air filter removes and transfers liquid droplets and foreign substance downstream of the compressor. The air tank reduces fluctuation of pressure of the compressed air. The pressure adjusting valve adjusts the pressure of the compressed air in the air tank. The air pressure members such as the electromagnetic valve and the nozzle control injection of the air.
Although advantageous, this configuration has a drawback. The air compressed by the compressor contains water. When the compressed air containing water is heated and cooled in the air pressure piping system, oversaturated water condenses into liquid droplets. In order to inject the compressed air into the atmosphere through the nozzle, the pressure of the compressed air in the air pressure piping system is reduced, causing adiabatic expansion and a decrease in the temperature.
This temperature drop generates liquid droplets, also known as drain fluid (hereinafter “drain”), in the air pressure pipes. If such drain accumulates in the air pressure pipes and the air is injected, the drain is injected from the nozzle, sticking to the fixing member and the recording medium, thus contaminating both the fixing member and the recording medium.
Furthermore, the drain in the air pressure pipes causes an operational problem and damage to the air pressure members such as the electromagnetic valve and the nozzle.
To address such a difficulty, a dehumidifier, also known as an air dryer, is provided downstream from the compressor in a device using a large compressor with an output of 1 kW or more. Various types of air dryer have been known. In one example of a known air dryer, moisture in high-temperature compressed air generated in the compressor is dehumidified by forced cooling , deliberately producing liquid droplets (drain). A water separator separates and discharges the drain outside the air pressure piping. Another method uses an absorbent material that absorbs moisture, or a hollow fiber filter that separates the moisture from the compressed air to discharge the moisture outside the air pressure piping.
Although advantageous, such known air dryers are generally expensive. Moreover, the air dryer using the air cooling method consumes relatively large amounts of power, and the air dryer using the hollow fiber filter requires high pressure of at least 0.2 MPa. By contrast, because generally-known image forming apparatuses only require a low pressure in a range of 0.05 to 0.2 MPa and a small flow rate to separate the recording medium from the fixing roller/belt, it is generally the case that the image forming apparatuses employ a small compressor with an output of 200 Watts or less, which does not adequately cool the moist air and produce drain before it arrives at the nozzles.
In view of the above, there is demand for a device capable of separating the recording medium with compressed air without contaminating the recording medium or other parts with drain.